1. Field of the Invention
This invention relates to an exhaust gas recirculation system for an internal combustion engine, such as a diesel engine.
2. Description of the Prior Art
It is known to provide an internal combustion engine with an exhaust gas recirculation (EGR) system to reduce the harmful emissions of oxides of nitrogen (NOx). The EGR system includes an EGR passage interconnecting the engine air intake passage and the engine exhaust passage. An EGR control valve is disposed in the EGR passage to controllably enable and disable the EGR, or to control the percentage of exhaust gas recirculated.
In the case of a diesel engine, the engine exhaust contains a significant amount of carbon soot or smoke, and the soot or smoke gradually accumulates in the exhaust passage as the engine ages. The soot or smoke accumulation usually causes the average pressure in the exhaust passage to increase. The increase of the exhaust passage pressure results in a gradual change in the percentage of exhaust gas recirculated. In this way, the characteristics of EGR change as the engine ages.